Devices for optically trapping small particles are better known as “optical traps” or “optical tweezers”. The technique relies on the forces created by one or more laser beams that are refracted by a microscopic object in order to trap, levitate and move that object. By focusing a laser beam though a microscope objective lens down to a very small spot (focal region), particles with high indices of refraction, such as glass, plastic, or oil droplets, are attracted to the intense regions of the beam and can be permanently trapped at the beam's focal region. Biologists are considerably interested in optical traps because minute forces can be measured with sub-picoNewton accuracy on the trapped object. Since such small forces are not accessible by conventional techniques such as scanning-force-microscopy, optical traps have become a major investigation tool in biology.
One (preferred) method to measure such forces includes capturing and analyzing the light after interacting with the particle and computing the change in momentum flux of the light due to interaction with the particle. Capturing all the light exiting the optical trap can be difficult, given that a single-beam trap needs highly marginal rays in order to trap efficiently, but even a high numerical-aperture (NA) lens may not accept all these rays when they have interacted with the particle and are deflected farther off the optic axis. In such a case, it can be difficult to capture and analyze all the light leaving the optical trap. Therefore, to address this issue, dual beam optical traps have been developed. Conventional counter-propagating beam optical traps have been used to trap particles, and measure extremely small transverse forces imparted on those particles. See for example, “Optical tweezers system measuring the change in light momentum flux”, Rev. Sci. Instrum., Vol. 73, No. 6, June 2002. Dual-beam traps are also better than single-beam traps for trapping particles with higher refractive indices.
One issue with optical traps is the alignment of the single or counter-propagating beams, so that the focal region(s) of the lens(es) are properly aligned with the sample (and each other for dual beam devices). For optical tweezers, two different methods are commonly used to move a laser-beam focus relative to samples in the objective focal plane: (1) move the sample chamber relative to the fixed beam-focus until the particle comes to the beam and is captured, or (2) translate the beam focus relative to the fixed sample chamber by changing the entrance angle of the laser beam into the back of the objective lens. The 2nd method has advantage of faster response time, as required for constant-position feedback that cancels Brownian motion in the trap. Beam-steering can be accomplished by several different methods, such as moving the lens, using a galvanometer to move a steering mirror, using an optic modulator to move the beam, translating the end of an optical fiber, etc. See for example, Svoboda and Block, Annu. Rev. Biophys. Biomol. Struct. v.23, pp. 247-285, FIG. 2 (1994).
Each of the conventional beam alignment techniques has its drawbacks. Moving the sample or lens is too slow to cancel Brownian motion. The galvanometer-mirror method is too large and complex. The acousto-optic modulator method is too large and complex, and has low light-transmission efficiency. Translating the delivery end of an optical fiber has potential advantage of simplicity, speed and high light efficiency, but suffers from an effect where the steered beam does not remain fixed at the back-focal-plane (BFP) of the objective lens. A beam steering method that does pivot about the BFP is described by Allersma et al. Biophys. J. v.74, pp. 1074-1085, FIG. 1B (1998). Unfortunately, that system uses the moving-lens “telescope” method, so the frequency response is lower than that required for Brownian-force noise cancellation.
There is a need for an optical alignment system and method, ideal for optical trap systems, that ensures and maintains the optical alignment of a light beam incident on an optical element such as focusing lens, while changing the incident angle of the light beam on that lens.